The present invention relates to an electric toothbrush which has a revolving motor shaft, a gear that converts the motion of rotation into a reciprocating stroke, and a slip-on brush which is removably attached to a handle segment. The gear connects to and imparts a reciprocating movement to a toothed rack disposed in the slip-on brush which, in turn, drives the rotatable tufts in the brush head.
Such an electric toothbrush is the subject matter of U.S. Pat. No. 4,827,550 issued to Graham on May 9, 1989, the entire disclosure thereof being incorporated herein by reference. The Graham toothbrush utilizes a bevel gear including an eccentric which is driven by a pinion of the motor shaft in the handle segment. A crank arm attaches the eccentric to a drive shaft which leads out of the handle segment. Thus, the eccentric and crank arm together translate the rotational movement of the gear to a reciprocating movement of the drive shaft. The drive shaft automatically couples with a toothed rack in the slip-on brush when said brush is slid on the handle segment. The reciprocating movement of the toothed rack imparted by the drive shaft generates in turn an alternating motion of rotation of the individual tufts in the brush head.
If one wants to attain an adequately long stroke of the toothed rack, so that the bristle tufts execute more than one rotation at each stroke, the drive shaft leading out of the handle segment must execute an equally long stroke. Since the eccentric is offset on the bevel gear by half the stroke, the diameter of the bevel gear would have to be enlarged. The diameter of the handle segment which houses the gear would thus also have to be enlarged, yet large diameter handles are difficult to manually hold and work. The problem on which the invention is based is to design an electric toothbrush of the aforementioned kind in such a manner that its gear can be arranged to convert the rotation of the electric motor into a reciprocating motion of a long stroke in a toothbrush with a relatively small diameter.